The current embedded base of data networks are based on IEEE 802 Local Area Networks, i.e., so-called "Legacy LANs". These Legacy LANs are so-called "connectionless" networks, because network entities exchange packets without establishment of a layer-2 connection. Many existing and emerging applications are designed to run primarily on Legacy LANs. These applications reside on top of so-called "layer-2" and "layer-3" protocols such as Medium Access (MAC) and Internet Protocol (IP), respectively. As is well known in the art, the layers referred to are those of the international standards organization (ISO) seven layer networking reference model.
A classical method for interconnecting different LANs is so-called "bridging and routing". Inter-LAN communication according to this technique is achieved through the use of an external router. A deficiency of this method is that, since it is based on a broadcast principle, and thus mimics shared-medium operations, all data packets are broadcast to all ATM destinations, thereby flooding the network with broadcast traffic. Another deficiency is that the broadcast nature of the technique virtually requires a mesh network between all bridges and ATM hosts within a LAN, and all inter-LAN traffic must pass through the router.
The ATM Forum has developed a better solution based on bridging called LAN Emulation (LANE), to interconnect Legacy LANs and native ATM hosts. LANE relies on a LAN Emulation Server (LES), which performs ATM-to-MAC address resolution, i.e., translation, and a Broadcast and Unknown Server (BUS), which performs data broadcast.
Generally speaking, LAN emulation clients (LECs) are ATM end-stations or ATM-capable bridges that are directly connected to an ATM network connecting Legacy LANs "behind" them to the ATM network. An LE.sub.-- ARP request allows a LEC to request an ATM address corresponding to a MAC address from a LES. The LE.sub.-- ARP request is defined in the ATM Forum's LAN Emulation Over ATM Specifications, Version 1.0, which is incorporated herein by reference and the contents of which are well known by those skilled in the art. It is noted that a so-called "Proxy LEC" represents multiple end-point addresses, e.g., the MAC addresses of several hosts. For a more detailed description of the definitions of LEC and proxy LEC reference may be made to the above-mentioned ATM Forum's LAN Emulation Over ATM Specifications, Version 1.0.
Broadcast data packets, such as a so-called "ARP.sub.-- Request", are forwarded to a BUS, which in turn broadcasts them to all LECs. "ARP.sub.-- Requests" are defined in Bell Communications Research (Bellcore) request for comments (RFC) 826, which is incorporated herein by reference. Also, data packets are sent to a BUS until a direct ATM connection is established to the target address within the Emulated LAN.
The communications between two Emulated LANs (ELANs) is done via an external router. Either the router has the ATM address of the destination LEC, or it requests the address from the LES serving the destination LEC. The router then builds an ATM connection to the destination LEC, and send thereto the data packet. Thus, disadvantageously, all inter-LAN packets must pass through the router, which may become a communications bottleneck, due to low throughput of conventional routers.
A third method builds upon LANE, but incorporates the routing function as well as bridging function into a so called multi-layer LAN switch. Fundamentally, there are three major functions associated with routing: 1) routing, i.e., determination of the layer-3 address of the next-hop-router along the path to the target address, 2) address resolution or translation, i.e., determination of a router's ATM address corresponding to its layer-3 address, and 3) data forwarding, i.e., relaying data packet from one port of the router to another port. A traditional router performs functions (1) and (3) while function (2) is required because an ATM connection must be established between adjacent ATM router hops or to the target ATM address. A multi-layer switch performs only function (3), i.e., data forwarding.
A route server is used to store the next-hop router's layer-3 address, and an address resolution protocol (ARP) server is used to resolve, i.e., translate, layer-3 addresses to ATM addresses. Sometimes these functions are merged into one server, a so-called "Route/ARP Server". With a multi-layer LAN switch, the intra-LAN communication is performed just as in LANE using the local LES and BUS of each ELAN. However, inter-LAN communication is different.
For example, if a host wants to talk to a host in a different ELAN, then the LEC for that host acts as a router, obtains either the a) destination's, or b) the next-hop-router's IP address from the route server, and obtains the corresponding ATM address from an ARP server. It then establishes an ATM connection directly to the destination LEC serving the host and sends the data packet.
This method is more efficient than using an external router since the external router hop is eliminated. Further efficiency is obtained by each multi-layer switch performing fast data forwarding both for layer-2 and 3 packets, while complicated route determination and address resolution functions are logically removed from the switches. Although the Route Server decouples the routing from data forwarding, it only can serve a few logical subnets, and is thus not suited to cover large number of logical subnets. As the number of subnets governed by a single route server increases the efficiencies provided with this approach greatly diminish, since several router hops become practically unavoidable.
Thus, the existing LANE and Route/ARP Server methods work effectively only for small scale local networks.
The forgoing problems with interconnecting ELANs are overcome, as described in U.S. patent application Ser. No. 08/402,235, filed Mar. 10, 1995 (Chang 1-1-1), which is incorporated herein by reference, by employing a so-called "ELAN contronect network" which is a separate network for interconnecting the servers of the sub-ELANs, where a sub-ELAN is a part of an ELAN having its own LES and BUS that may also be configured as a stand-alone ELAN. Each of the sub-ELANs is connected to the ELAN contronect network via a point-to-point connection-oriented connection, and the ELAN contronect network is configured to present itself to each of the servers of the sub-ELANs as clients thereof.
In one such system, the ELAN contronect network includes an address server and a broadcast/route server interconnected over a high-speed backbone. Also connected to the high-speed backbone is at least one LAN hub, which connects to at least one server of at least one of the ELANs.
The address server contains an address data base for performing address resolution, i.e., translation, between the at least two addresses of each ELAN end-point in response to requests for such translations. The broadcast/route server receives data packets for broadcast to a different sub-ELAN from which the packets originated and broadcasts the received data packets to at least one other sub-ELAN. The broadcast/route server also recognizes that a packet which is a broadcast packet at layer-2 is actually a request for an address resolution at layer-3. If so, the broadcast/route server collaborates with the address server to perform the necessary address resolution and insures that a response is sent only to the client originating the request.
Optionally, the ELAN contronect network also includes a configuration server and one or more multimedia servers, i.e., a multimedia server farm.